[Bug c/11663] New: gcc ignores the cast and emits a warning

[Bug c/11663] New: gcc ignores the cast and emits a warning

[jashepherd at usa dot net]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11663

           Summary: gcc ignores the cast and emits a warning
           Product: gcc
           Version: 2.96 (redhat)
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jashepherd at usa dot net
                CC: gcc-bugs at gcc dot gnu dot org

gcc -v
Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/2.96/specs
gcc version 2.96 20000731 (Red Hat Linux 7.2 2.96-116.7.2)

/*
  gcc ignores the cast and emits a warning 

  gcc bug.c -funsigned-char
  bug.c: In function `main':
  bug.c:13: warning: comparison is always true due to limited range of data type

*/

char trans_ind = 3;
int main(int argc, char *argv[])
{
   if ((short)trans_ind >= 0 && (short)trans_ind <= 3)
      printf("Yes\n");
   else
     printf("No\n");    

   return(0);
}

[Bug c/11663] gcc ignores the cast and emits a warning

[pinskia at physics dot uc dot edu]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11663


pinskia at physics dot uc dot edu changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
           Keywords|                            |diagnostic
         Resolution|                            |INVALID
            Summary|gcc ignores the cast and    |gcc ignores the cast and
                   |emits a warning             |emits a warning
   Target Milestone|3.4                         |---


------- Additional Comments From pinskia at physics dot uc dot edu  2003-07-24 18:14 -------
The warning is correct because converting unsigned char to short will always be positive 
or zero.  GCC is keeping the range information for trans_ind, if you want to convert 
unsigned char to signed short with trans_ind > 127 being negative add a cast to signed 
char first.

[Bug c/11663] gcc ignores the cast and emits a warning

[jashepherd at usa dot net]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11663



------- Additional Comments From jashepherd at usa dot net  2003-07-24 18:54 -------
Subject: Re:  gcc ignores the cast and emits a warning

A cast to (short) has to assume the constraints of the cast datatype
not the original data type.

The result of the entire if test

    if ((short)trans_ind >= 0 && (short)trans_ind <= 3)

can never be 'always true' regardless of whether the datatype
is char, unsigned char or short.

only in the case where the test is

    if ( trans_ind >= 0 )

would the warning be valid.







At 2003-07-24 18:14 +0000, you wrote:
>PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.
>
>http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11663
>
>
>pinskia at physics dot uc dot edu changed:
>
>            What    |Removed                     |Added
>----------------------------------------------------------------------------
>              Status|UNCONFIRMED                 |RESOLVED
>            Keywords|                            |diagnostic
>          Resolution|                            |INVALID
>             Summary|gcc ignores the cast and    |gcc ignores the cast and
>                    |emits a warning             |emits a warning
>    Target Milestone|3.4                         |---
>
>
>------- Additional Comments From pinskia at physics dot uc dot 
>edu  2003-07-24 18:14 -------
>The warning is correct because converting unsigned char to short will 
>always be positive
>or zero.  GCC is keeping the range information for trans_ind, if you want 
>to convert
>unsigned char to signed short with trans_ind > 127 being negative add a 
>cast to signed
>char first.
>
>
>
>------- You are receiving this mail because: -------
>You reported the bug, or are watching the reporter.

[Bug c/11663] gcc ignores the cast and emits a warning

[pinskia at physics dot uc dot edu]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11663



------- Additional Comments From pinskia at physics dot uc dot edu  2003-07-24 19:02 -------
But keeping the range information (because the cast will not change the sign in this case) is still 
valid so the warning still stands.